Recently, there is an increasing tendency to employ base metals such as copper, nickel and the like as a material for electrodes or conductor patterns of thick film circuits. Such thick film circuits are generally produced, for example, by respectively printing a conductive pattern of base metal paste and a resistive pattern of resistive paste on substrates, and then firing the same in a non-oxidizing or reducing atmosphere to prevent the conductor patterns from oxidation. It is therefore required to use resistive paste with a high resistance to reduction.
To this end, there have been proposed a variety of resistive pastes generally comprising a conductive material such as metal hexaboride and a nonreducible vitreous binder suspended in an organic vehicle. For example, Japanese patent published No. 59-6481 and Japanese patent laid open Nos. 55-277700 and 55-29199 disclose resistive paste containing lanthanum hexaboride as the conductive material, and a nonreducible glass frit of calcium boroaluminate, barium borosilicate or calcium borosilicate glass as the vitreous binder.
Such a resistive paste can be applied to production of thick film circuits comprising resistors with sheet resistivity ranging from 10 .OMEGA. to 10 K.OMEGA.. However, such a resistive paste does not provide repeatable results since the sheet resistivity of the resistors produced varies greatly with a slight change of the ratio of glass frit to metal hexaboride. In addition, it is impossible with such resistive pastes to produce thick film resistors with a sheet resistivity of more than 10 K.OMEGA. since the sheet resistivity increases abruptly and becomes more than 1 G.OMEGA. when the ratio of the glass frit to metal hexaboride exceeds 50 wt% slightly. Further, the thick film resistors with a sheet resistivity of not less than 10 K.OMEGA. possess a temperature coefficient of resistance of not less than -1000ppm/.degree. C., thus making impossible to put them into practical use.